|
In computer science, self-modifying code is code that alters its own instructions while it is executing - usually to reduce the instruction path length and improve performance or simply to reduce otherwise repetitively similar code, thus simplifying maintenance. Self modification is an alternative to the method of "flag setting" and conditional program branching, used primarily to reduce the number of times a condition needs to be tested. The term is usually only applied to code where the self-modification is intentional, not in situations where code accidentally modifies itself due to an error such as a buffer overflow. The method is frequently used for conditionally invoking test/debugging code without requiring additional computational overhead for every input/output cycle. The modifications may be performed: * only during initialization - based on input parameters (when the process is more commonly described as software 'configuration' and is somewhat analogous, in hardware terms, to setting jumpers for printed circuit boards). Alteration of program entry pointers is an equivalent indirect method of self-modification, but requiring the co-existence of one or more alternative instruction paths, increasing the program size. * throughout execution ('on-the-fly') - based on particular program states that have been reached during the execution In either case, the modifications may be performed directly to the machine code instructions themselves, by overlaying new instructions over the existing ones (for example: altering a compare and branch to an unconditional branch or alternatively a 'NOP'). In the IBM/360 and Z/Architecture instruction set, an EXECUTE (EX) instruction ''logically'' overlays the second byte of its target instruction with the low-order 8 bits of register 1. This provides the effect of self-modification although the actual instruction in storage is not altered. ==Application in low and high level languages== Self-modification can be accomplished in a variety of ways depending upon the programming language and its support for pointers and/or access to dynamic compiler or interpreter 'engines': * overlay of existing instructions (or parts of instructions such as opcode, register, flags or address) or * direct creation of whole instructions or sequences of instructions in memory * creating or modification of source code statements followed by a 'mini compile' or a dynamic interpretation (see eval statement) * creating an entire program dynamically and then executing it 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Self-modifying code」の詳細全文を読む スポンサード リンク
|